Pat, additional offline storage would be accomplished by placing a second synology at an off-site location and syncing them. Whether you use post to send drives, or tubes to send bits, they both serve the same purpose, a place to store your off-site backup.
The only reason to use post is if the amount of data is so huge you can’t transfer over internet. Carrier pigeon has fantastic bandwidth, but lots of latency.
If you prefer a true cold store off site, you can use a service for that. Takea synology snapshot, transfer to another synology, and then take those disks and mail them.
You’re most likely better served using something like box.net or aws cold store though.